In the hydrocarbon exploration and production industry, it is common to use perforating guns to form fluid communication paths (“perforations”) in a subterranean formation between a hydrocarbon reservoir and a drilled wellbore that traverses the reservoir. The communication paths enable the inflow of production fluids into the wellbore, and enable the delivery of stimulation fluids to the formation, for example during hydraulic fracturing operations. Typically perforation methods are applied to cased hole wellbores, which include a casing string cemented within the wellbore to increase the integrity of the wellbore and provide a flow path to surface for fluids produced from the formation. The perforations extend through the steel casing, the cement on the outside of the casing, and into the formation. Similar methods are used in the fields of water and geothermal exploration.
It is conventional to form the perforations by placing a perforating gun which incorporates shaped charges inside the casing string next to the formation to be perforated. A typical perforation gun comprises a charge carrier and a series of shaped charges connected to a detonator by a detonation cord. The perforation gun forms a part of a tool string which is conveyed into the wellbore by a flexible line, drill string, coiled tubing, or other conveyance. Commonly, flexible line such as wireline, electric line or slickline is used to convey the perforating gun to the required depth.
With the charge carriers located in the interval to be perforated, the shaped charges are detonated to generate high-pressure streams of particles in the form of jets. The jets penetrate through the casing, the cement and into the formation.
Challenges associated with the successful design and operation of perforation guns include gun survivability during the perforating operation, and the effectiveness of perforations during hydrocarbon production or stimulation operations.
Gun survivability concerns the capability of the gun to retain its mechanical integrity during and after detonation of the shaped charges such that it can be successfully retrieved from the wellbore without damaging the casing or causing debris to left downhole. The survival risk associated with a perforation gun is dependent on a number of different factors, including the magnitude of the shaped charges, the design and materials of the charge carrier and tool string, the phasing of the shaped charges, or their proximity (or shot density). Where perforation in a single or narrow phasing plane is desirable, as may be the case in sand control or hydraulic fracturing applications, it is common to account for increased survival risk by increasing the shot-to-shot spacing, increasing the mechanical rating of the perforating gun (for example by specifying high grade or high integrity materials) or reducing the shaped charge explosive capacity. Undesirable compromises of these factors may include a reduction in perforation density, increased tool cost, and/or a reduction in perforation penetration depth.
U.S. Pat. No. 5,673,760 describes a configuration which is said to reduce gun survival risk by arranging groups of charges in the same cylindrical plane and closely packing the charges. Charges within a group are detonated symmetrically. This system is claimed to reduce gun swelling and reduce the amount of debris that escapes the perforating gun.
Various factors contribute to the effect of the perforations on the productivity of the well or the success of a fracturing operation. These include depth and effective diameter of perforation tunnels. The pressure condition within the wellbore during the perforation process also has a significant impact on the efficiency of the perforations.
The perforation is formed in an overbalanced pressure regime if the hydrostatic pressure inside the casing is greater than the reservoir pressure. Perforating underbalanced is when the perforation is formed under conditions in which the hydrostatic pressure inside the casing is less than the reservoir pressure. Underbalanced perforating has the tendency to allow the reservoir fluid to flow into the wellbore, and is generally preferable as the influx of reservoir fluid into the wellbore tends to clean up the perforation tunnels and increase the depth of the clear tunnel of the perforation.
It has been found, however, that even when perforating is performed underbalanced, the effective diameter of the perforation tunnels can be small as the jet of metallic particles that creates the perforation tunnels is highly concentrated. Due to the small diameter of the perforation tunnels, the volume of the perforation tunnels is also small. In addition, it has been found that even when perforating is performed underbalanced, the surface of the perforation tunnels may have reduced permeability compared to the virgin rock.
One technique for generating perforations with improved inflow characteristics is to use groups or banks of convergent or focused shaped charges. U.S. Pat. No. 3,347,314, U.S. Pat. No. 7,303,017, U.S. Pat. No. 7,172,023 and U.S. Pat. No. 7,409,992 are examples of perforation devices which used convergent charge groups to create an enhanced perforation cavity. The cavities formed are said to be of relatively large volume with high permeability to enhance productivity.
Disadvantages of convergent charge configurations of the types described in U.S. Pat. No. 3,347,314, U.S. Pat. No. 7,303,017, U.S. Pat. No. 7,172,023 and U.S. Pat. No. 7,409,992 result from the requirement for the shaped charges to operate in functional groups. This places restrictions on shaped charge placement and phasing, which can have an adverse effect on production flow geometry in a radial direction around the wellbore. It may be desirable for the convergent charges within a group to be closely spaced to one another for proper interaction of the jets generated by the convergent charges, but gun survivability issues require compromises in shot density, charge capacity or increased material costs in order to reduce the survival risk.